Recently, advances have been made which strike a more desirable balance between maintaining fuel efficiency and reducing the percentage of particulate emissions in fuels through the use of blends of petroleum based fuel with alkyl esters of the fatty acids contained in vegetable oils or animal fats. These alkyl esters are commonly referenced to as “biodiesel”. Substantially pure alkyl esters, such as methyl or ethyl esters of fatty acids, are generally preferred in biodiesel over the use of the vegetable oils and animal fats themselves because the alkyl esters have a viscosity that is more appropriate to diesel fuel. Through the use of these fuel blends, researchers have attained reductions in particulate emissions from diesel engines [1]. The production of biodiesel has received also extensive interest as a result of this fuel's desirable renewable, biodegradable, and nontoxic properties [2].
These fatty acid alkyl esters can be prepared by the transesterification of triglycerides in vegetable oils with short-chain alcohols (e.g., methanol and ethanol) using homogeneous alkali catalysts such as alkoxides. For example, soy diesel (methyl soyate) is made commercially by an energy and labor-intensive process wherein soybean oil is reacted with methanol at 140-150° F. (sometimes under pressure) in the presence of sodium methoxide. Isolation of the desired methyl soyate from the highly caustic (toxic) catalyst and other products such as glycerol, involves a precise neutralization process with strong acids, such as hydrochloric acid (HCl), and extensive washes with water to remove the resulting sodium chloride (NaCl) salt. Also, because of glycerol's high boiling point, it must be separated from the sodium chloride salt by vacuum distillation in an energy intensive operation. As more alkyl soyates with different alkyl functional groups, such as ethyl and isopropyl soyates, are being rapidly developed to meet the growing needs of various applications, the level of difficulty in separating the corresponding catalysts, e.g., sodium ethoxide and sodium isoproxide catalysts, respectively, will unavoidably escalate due to the increasing solubility of these basic catalysts in the reaction mixture. Therefore, biodiesel is currently not cost competitive with conventional diesel fuel.
To improve the economic outlook of biodiesel and alkyl esters in general, the feedstock selection becomes critical. In particular, feeds containing high free fatty acid content, such as found in beef tallow or yellow grease, are significantly less expensive than vegetable oils, such as soybean or rapeseed oil [3]. These high free fatty acid feedstocks present significant processing problems in standard biodiesel manufacture since the free fatty acid is saponified by the homogeneous alkali catalyst that is used to transesterify triglycerides leading to a loss of catalyst as well as increased purification costs [4].
One approach for improving the processing of high free fatty acid oils and fats is to first esterify the free fatty acids to alkyl esters in the presence of an acidic catalyst such as a mineral acid. The pretreated oils in which the free fatty acid content is lowered to no more than 0.5 wt % can then be processed under standard transesterification reaction conditions [5]. This pretreatment step has been successfully demonstrated using sulfuric acid [6]. Unfortunately, use of the homogeneous sulfuric acid catalyst adds neutralization and separation steps, as well as the esterification reaction, to the overall process.
Surfactant-templated mesostructured materials have received a great deal of attention as potential catalysts, sensors and adsorption agents owing to their combination of extremely high surface areas and ordered, flexible pore sizes. For example, mesoporous sieves of the type MCM-41 are prepared by thermal treatment of silaceous gels formed by the polymerization of alkoxysilanes around surfactant micelle templates in aqueous base, followed by removal of the surfactant to yield a matrix comprising fine pores in a cylindrical array. The physical and chemical properties of these mesoporous materials can be modified by incorporating functionalized organic groups, either by grafting on the preformed mesopore surface or by co-condensation using functionalized substituted trialkoxy silanes during synthesis [7-23]. Not only can such catalysts be easily separated from the products and recycled, but their large surface area (>700 m2/g), defined pore structure, tunable pore diameter (2-10 nm) and narrow pore size distribution allow for precise regulation of the mass-transport properties that are crucial for many chemical transformations. Several recent studies have demonstrated that the selectivity and reactivity of MSN-supported catalysts can be enhanced by introducing multiple functional groups onto the 3D controlled mesoporous surface [8-11, 14, 16].
For example, organic-inorganic hybrid mesoporous silicas formed by co-condensation with thio-containing silanes, followed by oxidation of the SH groups yield pores functionalized with sulfonic acid groups. The direct co-condensation synthesis technique in which the mesostructure and functional group are simultaneously introduced, appears to be a desirable route for incorporating functional groups because it has been shown that it increases the concentration of the sulfonic groups in the mesoporous silica relative to post-formation grafting [24]. One approach demonstrated previously involves one-step synthesis based on the simultaneous hydrolysis and condensation of tetraethoxysilane (TEOS) with 3-(mercaptopropyl)trimethoxysilane (MPTMS) in the presence of template surfactant using in situ oxidation of the thiol groups with H2O2. Melero et al. has shown that the acid strength of the sulfonic groups in the mesoporous materials can be adjusted by choice of the organosulfonic precursor [25].
For example, mesoporous catalysts containing sulfonic acid groups and, optionally internal methyl groups have been reported to be efficient catalysts in the esterification of glycerol with fatty acids, where high yields of mono-esters are obtained [26].
Lin et al. (U.S. Pat. No. 7,122,688) discloses a method for transesterifying glyceride containing oils by combining a glyceride-containing vegetable or animal oil such as soy oil, a C1-C3 alcohol, and an acidic mesoporous silicate under conditions so that the mesoporous silicate catalyzes formation of the corresponding fatty acid (C1-C3) alkyl ester and optionally, glycerol, and wherein the silicate comprises sulfonic acids, sulfinic acids, phosphoric acids, phosphinic acids, boronic acids, selenic acids and mixtures thereof, linked to the silicate matrix by inert organic groups.
However, a continuing need exists for a simple method to form (lower)alkyl esters of fatty acids in the environment of triglyceride-containing feedstocks, particularly greases or fats that contain large amounts of free fatty acids mixed with triglycerides.